A semiconductor light-emitting element in an optical coupling device typically emits an optical signal at a wavelength in a range of a red to infrared light. A light receiving element in the optical coupler then converts the optical signal into an electric signal. The light receiving element can be, for example, a silicon (Si) photodiode. Therefore, it is possible to transfer a signal between an input terminal and an output terminal that are electrically insulated from each other. Optical coupling devices are widely used in industrial and telecommunication applications.
The red light to the infrared light are emitted from a light emitting layer which is formed of AlGaAs, InGaAlP, or the like. However, in the light emitting layer which is formed of materials including Al, a micro-crystal defect, which is mechanically introduced, or a fine point defect in a hetero-interface, which is introduced when executing an epitaxial growth, becomes a non-light emission center due to stresses generated when the device is operating. These non-light emission centers cause light absorption and become recombination centers, and thus an optical output may be decreased due to the growth and spread of the crystal defect in some cases.